Modulation systems and methods are widely used in transmitters to modulate information including voice and/or data onto a carrier. The carrier may be a final carrier or an intermediate carrier. The carrier frequency can be in UHF, VHF, RF, microwave or any other frequency band. Modulators also are referred to as “mixers” or “multipliers”. For example, in a wireless communications terminal such as a mobile radiotelephone, a modulator can be used for the radiotelephone transmitter.
FIG. 1 illustrates a conventional IQ modulator. As shown in FIG. 1, an IQ modulator 110, also referred to as a “quadraphase modulator” or a “quadrature modulator”, includes a quadrature splitter 120, also known as a 90° phase shifter, and a pair of multipliers 116a, 116b coupled to the quadrature splitter. A controlled oscillator 115, such as a Voltage Controlled Oscillator (VCO), is coupled to the quadrature splitter 120 to produce 90° phased shifted oscillator signals. In-phase (I) data 111a and quadrature-phase (Q) data 111b are coupled to a respective multiplier or mixer 116a, 116b. Digital input data is converted to analog data by I Digital-to-Analog Converter (DAC) 114a and Q DAC 114b, respectively. The outputs of the respective DACs 114a and 114b are applied to the respective low pass filters 112a and 112b to provide the respective I and Q data inputs 111a and 111b. The modulator 110 modulates the input data on a carrier by summing the outputs of the multipliers 116a, 116b at a summing node 118. The modulated carrier 113 is amplified by a power amplifier 122 and transmitted via an antenna 124.
In modern wireless communications, wireless communications terminals such as mobile radiotelephones continue to decrease in size, cost and/or power consumption. In order to satisfy these objectives, it generally is desirable to provide IQ modulation systems and methods that can provide high power modulation while reducing the amount of battery power that is consumed. Unfortunately, the power amplifier 122 of an IQ modulator may consume excessive power due to efficiency limitations therein. More specifically, it is known to provide a linear class-A or class-AB power amplifier 122 that may have efficiencies as low as 30 percent or less. Thus, large amounts of battery power may be wasted as heat. Moreover, the noise figure of a conventional IQ modulator may be excessive so that high cost Surface Acoustic Wave (SAW) filters may need to be used.
FIG. 2 illustrates other conventional modulation systems. As shown in FIG. 2, I-data and Q-data is modulated on an Intermediate Frequency (IF) signal supplied by a controlled oscillator such as a voltage controlled oscillator 202 by applying the I-data and Q-data and the output of the IF voltage controlled oscillator 202 to an IQ modulator 204. The output of the modulator is then bandpass filtered by an IF bandpass filter 206. A local oscillator 212 and an up-conversion mixer 214 are used to up-convert the output of the bandpass filter 206 to a desired radio frequency. The output of the up-conversion mixer 214 is bandpass filtered by a radio frequency bandpass filter 216 to reduce noise and spurious levels. The filtered signal is then amplified using a variable gain amplifier 222 to provide the appropriate signal level to a power amplifier 226 which delivers the signal to an antenna 232 via a duplex filter 234. Additional RF bandpass filtering 224 may be used between the variable gain amplifier 222 and the power amplifier 226.
FIG. 3 is a block diagram of other conventional modulation systems wherein like elements to FIG. 2 are labeled with like numbers. The approach shown in FIG. 3 is similar to that of FIG. 2 except the IF signal is up-converted to the RF band first and then modulated in the IQ modulator 204.
Unfortunately, in either of the conventional approaches of FIGS. 2 or 3, the IQ modulator 204, up-conversion mixer 214 and/or the variable gain amplifier 222 may generate significant amounts of additive noise and spurious levels which may need to be filtered before the signal reaches the power amplifier 226. Systems of FIGS. 2 and 3 also may suffer from high current consumption and may need to use an excessive number of filters to meet the desired output spurious level and desired noise level.
It also is known to separately modulate the amplitude and phase of an input signal using an “rTheta” technique. In the rTheta technique, the phase is modulated at the oscillator, and the amplitude is modulated at the power amplifier stage. Unfortunately, the rTheta technique may require the oscillator phase locked loop to support the phase modulation bandwidth. With wide bandwidth radiotelephone signals such as TDMA and CDMA signals, it may be increasingly difficult to provide the requisite bandwidth in the oscillator phase locked loop.